Kindle jumps onto social network bandwagon with Twitter and Facebook features
I don't know how useful these features are going to be to Kindle owners, but they're extremely useful as free advertising for Amazon. After seeing so many "hey, I'm using my Kindle!" messages on social networks, you may just give in and buy one. If Amazon really wants to make books social, they should buy Goodreads. The closest the Kindle gets in this new update is a feature that shows you which passages in a book were most frequently bookmarked by others.
As far as non-social features, Kindle 2.5 introduces a password lock, new fonts, and the ability to organize your books into collections. It also adds the much-needed ability to pan and zoom within PDFs.